Means for compensating for temperature changes in piezo-electric crystal devices



May 14, 1935. 'r. J. SCOFIELD 2,001,217

MEANS FOR COMPENSATING FOR TEMPERATURE CHANGES IN PIEZO ELECTRIC CRYSTALDEVICES Filed July 21, 1933 FIG..1.

ATTORNEY 4 Patented May 14, 1935 UNITED STATES DIEANS FOR COMPENSATINGFOB TEDIPEB- ATURE CHANGES IN PIEZQ-ELECTRIO CRYSTAL DEVICES Theodore J.Scofield, Jackson, Mich., assignor,

by mesne assignments, to Radio Corporation of America, New York, N. Y.,a corporation of Delaware Application July 21, 1933, Serial No. 681,469

2 Claims. (01. 171-327) This invention relates to piezo electric crystaldevices for control of oscillator frequencies, tuning circuits andkindred uses. 1 I It is weliknown that the oscillating frequency of thequartz crystal decreases with increase in temperature. It is also wellknown that the crystal oscillates best when not under compression, infact, a space of about one-thousandth of an inch is provided at the sideof the crystal to give it freedom in oscillations. If this space isdecreased, for example, by increase in temperature, the frequency ofoscillation of the crystal is lowered.

On account of these two characteristics of the quartz crystal, increasein temperature of the crystal and the mounting decreases the free spaceadjacent the crystal and lowers its period of oscillation and increaseintemperature of the crystal itself lowers its period. Lowering of thetemperature of the crystal and associate parts has a reverse eifect.

It accordingly has been the practice to mount crystals in enclosureswith automatic means for holding the temperature constant within verynarrow limits, particularly where the frequency of radio transmittersare being controlled by the crystals. The maintaining of a constanttemperature around a piezo electric crystal is expensive in apparatusand maintenance and it is desirable to obviate the use of suchtemperature controls.

In the application of Stuart W. Seeley, Serial No. 681,487, filed July21, 1933, means have been described for automatically compensating fortemperature changes in a crystal control device.

It is an object of my invention to improve upon the invention in saidapplication so that the same crystal mounting and associated parts maybe used for a plurality of crystals having different temperaturecoeflicients or different thickness.

I have illustrated the invention in the drawing in which:

Fig. 1 is a sectional elevation of a crystal control device embodyinginvention, the section being taken on the line l-l of Fig. 2.

Fig. 2 is a plan view of the device shown in Fig. 1 with the exceptionthat a part of the devices sectioned on the line 2-4 of Fig. 1.

Referring to the drawing, a metal cylinder l is secured to an insulatingbase 3 by any means such as screws 4. On the base 3 is located ametallic plate 5 and on this plate rests the quartz crystal 6. Above thecrystal is a metal top plate 1 having a screw shank 8 threaded through acollar or boss 8' on the top piece of the cylinder. When the shank isthreaded in and out oi the boss the disc I can be moved toward and awayfrom the crystal 6 any desired amount.

If the disc I is adjusted for a certain normal temperature of oneone-thousandth of an inch or any other desired, spacing and thetemperature increases, the crystals oscillating frequency will lower andthis will be augmented by decrease of the space through the expansion ofthe crystal. To counteract this double effect the spacing between thecrystal 6 and the plate I should increase above the spacing previouslyset. To automatically increase this spacing the cylinder is made ofdifferent material from the screw 8 so that the combined expansion issuch as to increase the free space between the crystal and the disc 1.One mayselect various metals for accomplishing the purpose, the metalschosen depending upon the lengths of the metallic parts. In general, itmay be said that the cylinder I should be made of metal that has agreater coefficient of expansion than the screw 8 so that the spacebetween the crystals 6 and the plate I will increase with an increase oftemperature. Various materials may be chosen for these two parts,depending upon the design and the apparatus. One may select suchmaterials from standard lists of metals but a suitable choice may bemade from the following:

Electrical connections will be made in any desired way between electricplates 5 and l and the other parts of the circuit. These are not shownon the drawing.

The invention so far described in detail is that of Stuart W. Seeleydisclosed in the above mentioned application. In my improvement I sodesign the device that various crystals may be used having differenttemperature co-efllcients or different thicknesses or other differentcharacteristics. Instead of tapping the collars 8' to thread with theshank 8 I make the bore 9 therethrough without threads. The shank 8snugly fits the bore and is supported by pointed opposed pairs of setscrews l0, ll, arranged at various distances from the top plate of frameI. Only one of these pairs of set screws will have the points thereforeengaged between the threads of the adjusting screws 8. The others willbe threaded out of engagement with such adjusting screws. 7

Let us suppose that with a given crystal 6 the set screws second fromthe top shown in Fig. 1 will be adjusted so that the points engage theshank 8 and the length between such screws and the plate I is such as tocause proper compensation for temperature changes. If this crystal isremoved and some other crystal inserted that has difierent thicknessesor temperature characteristic, the coacting screws H), II of Fig. 1would be adjusted to inactive position and some other opposed pair ofscrews In, H, would be threaded into engagement with the screws 8 sothat compensation would result. By trial the proper pair of set screwsmay be selected to secure correct compensation.

While I have shown the particular means of changing the length of theadjusting shank 8 or equivalent part without changing the air gapbetween the top plate I and the crystal 6, it will be understood thatvarious other modifications may be devised for accomplishing the samepurpose without departing from the spirit of the invention.

Having described my invention, what I claim is:

1. In crystal controls, a crystal, a conducting plate on each side ofthe crystal, and means to change the spacing between said plates inresponse to temperature changes to maintain the oscillations of thecrystal constant as its temperature changes, and means to manuallyadjust the first mentioned means without substantially varying saidspacing.

2. In crystal controls, a crystal, a plate adjacent to one side of saidcrystal, a second plate adjacent to the other side of the crystal, alink supporting one plate and extending past the other plate, a secondlink connected to the first link and extending toward the second plate,said links having different coefiicients of expansion and being of suchlength as to maintain the frequency characteristic of the crystalconstant as the temperature changes, and means to manually alter thelength of said links without substantially changing the relativedistance between said plates.

THEODORE J. SCOF'IELD.

